1. Field of the Invention
The present invention relates generally to an apparatus for calculating a degree of white balance adjustment for a picture, and more particularly to an apparatus for calculating a degree of white balance adjustment for a picture which is used for a color image outputting apparatus such as a color video printer, a color copy machine or the like and is appropriate to perform a hard-copy of a picture.
2. Description of the Related Art
In many types of video cameras and video movies, an auto white balance adjustment function is provided, and the white balance adjustment is performed on the assumption that an integrated value (or an average value) of color difference signals for a picture in which an appropriate white balance is kept becomes zero. However, in cases where a chromatic-colored object is pictured to occupy a wide region of the picture, the assumption is not necessarily correct. Therefore, when the auto white balance adjustment is performed, a picture quality often deteriorates. For example, a picture adjusted in auto white balance is faded.
2.1. Previously Proposed Art
To solve this problem, many auto white balance adjusting methods in which many types of improvement are performed are developed and disclosed. For example, in an adjusting method disclosed in the Published Unexamined Japanese Patent Application No. H3-274884 (274884/1991), a plurality of color difference signals of pixels which have colors near to white and are placed in a rectangular region along a black-body radiation curve in a color space are integrated to perform the white balance adjustment. Also, in an adjusting method disclosed in the Published Unexamined Japanese Patent Application No. H3-250992 (250992/1991), a photographed picture is partitioned into 8.times.8 regions, an integrated value of color difference signals for each of the regions is estimated, and the adverse influence of a high chromatic-colored object pictured in a region on a white balance adjustment for all regions of the photographed picture is reduced. However, these types of improvement are not sufficient to perform the auto white balance adjustment with high accuracy without any deterioration of picture quality.
Also, a picture quality of an image printed by a video printer is influenced by the quality of picture signals input to a television, a video tape recorder, a laser disk, a video movie or the like. Therefore, it is required to consider that the picture signals in which any white balance adjustment is not performed because of a manual photographing are reproduced by the video printer. Even though the auto white balance adjustment is performed for the picture signals in a photographing apparatus, the picture signals in which the auto white balance adjustment is not sufficient or is erroneously performed are reproduced by the video printer. Therefore, the auto white balance adjustment sufficiently performed with high accuracy is required of a color image outputting apparatus.
2.2. Problems to be Solved by the Invention
A conventional video printer having an auto white balance function is disclosed in the Published Unexamined Japanese Patent Application No. H2-298194 (298194/1990). In this Application, a degree of white balance adjustment is calculated on the basis of an integrated value of color difference signals, in the same manner as that in the white balance adjusting method for the video camera. However, there is a problem that the white valance adjustment is erroneously performed because of the existence of a high chromatic object in the same manner as the white balance adjustment in the video camera.
Also, in the Published Unexamined Japanese Patent Application No. H4-323994 (323994/1992), a chromaticity threshold is used to remove pixels relating to a high chromaticity and to select pixels relating to low chromaticity for the purpose of avoiding adverse influence of the high chromaticity, and the white balance adjustment is performed by using information relating to the low chromaticity. However, in cases where the white balance is not kept in a picture, an achromatic color used as a basis of chromatic colors cannot be defined, and chromaticity cannot be determined with high reliability. That is, an appropriate white balance adjustment cannot be necessarily performed.
Therefore, in cases where the white balance adjustment is performed on the basis of an integrated value of color difference signals, it is important how the adverse influence of a high chromaticity region in a picture is removed to calculate a degree of white balance adjustment without any erroneous adjustment.
Also, in cases where a white region slightly colored exists in a picture, it is preferred that the white balance adjustment be not performed on the basis of an integrated value of color difference signals but the white balance adjustment be performed on the basis of the white region.
Accordingly, it is preferred that the white balance adjustment based on an integrated value of color difference signals or the white balance adjustment based on a white region detected be selected by considering features of a color distribution of a picture.